A DIAMONDIFEROUS LHERZOLITE FROM THE PREMIER DIAMOND MINE, SOUTH AFRICA K.S. (Fanus) Viljoen and Rene Dobbe GeoScience Centre, De Beers Consolidated Mines, South Africa INTRODUCTION PETROGRAPHY This paper reports on the discovery of a diamondiferous The xenolith is altered with pervasive serpentinisation peridotite from the Premier diamond mine. Such of constituent minerals (Figure 1). Garnets are generally xenoliths are surprisingly rare on the Kaapvaal Craton, partially kelyphitised. They range in size up to 2mm. with 4 specimens having been decribed from Finsch These occur interstitially with serpentine pseudomorphs (Shee et al 1982; Viljoen et al 1992), 8 from Roberts after possible olivine and/or orthopyroxene. The Victor (Viljoen et al 1994), and one from Mothae in pervasive alteration complicates petrography and it is Lesotho (Dawson and Smith 1975). A number of not possible to accurately determine volume diamond-bearing peridotitic garnet macrocrysts have percentages of these two minerals, assuming that both also been recognised at the Newlands kimberlite are present. Other minerals typical of peridotites such (Daniels et al 1995). as clinopyroxene and accessory spinel are not seen. The The x-ray diamond recovery circuit (the Sortex Plant) at xenolith texture is coarse (i.e. not sheared). the Premier diamond mine in South Africa typically Inclusions in Diamond 16 generates about 3 specimens of diamond partially Premier Xenolith enclosed in kimberlite each day. These are sent to the Other Xenoliths sorthouse at the mine where a small press is used to 14 liberate the diamonds from the kimberlite matrix. Maggie Mahlase, a diamond sorter, recognised the 12 importance of the specimen when crushing revealed numerous diamonds. At least 50 diamonds, ranging in 10 size from less than 1 mm to approximately 2mm, are available for study, along with numerous cm-sized (wt%) 3 8 fragments of the original xenolith (many of which still O 2 contain diamonds in-situ). r 6 C 4 2 0 01234567 CaO (wt%) Figure 2: Distribution of CaO and Cr2O3 in peridotitic garnets occurring as inclusions in diamonds from Premier as well as the diamond-bearing peridotite from Premier. Also shown are garnets from other diamond- Figure 1: Photomicrograph of the diamond-bearing bearing peridotites and diamond-bearing macrocrysts in peridotite from Premier. Rounded garnets are set in a Southern Africa. The Premier xenolith garnets plot coarse matrix of serpentine pseudomorphs after within the lherzolite field as defined by the 85% line of possible olivine and orthopyroxene. Gurney and Switzer (1973). 8th International Kimberlite Conference Long Abstract 1 MINERAL CHEMISTRY Infrared analysis of the xenolith diamonds show nitrogen contents generally lower than 500 ppm and Electron microprobe analysis of the cores of 11 variable nitrogen aggregation state, from 10% to 80% individual garnets show extremely limited chemical of the highly aggregated 'B' form. When plotted on a variability between grains. They contain, on average, nitrogen aggregation diagram a well defined trend of 0.24 wt% TiO2, 8.5 wt% Cr2O3 and 6.6 wt% CaO. The increasing nitrogen aggregation state with increasing Cr and Ca values are consistent with a lherzolitic nitrogen content is observed (Figure 4). This equates to paragenesis (Figure 2). Garnets occurring as inclusions a calculated mantle residence temperature on the order in diamonds from the Premier Mine (Richardson et al of 1200ºC at an assumed mantle residence time of 750 1993) as well as garnets from diamond-bearing My for lherzolitic diamonds at Premier (Richardson et peridotites and diamond-bearing macrocrysts from the al 1993). Kaapvaal craton are characterised by a range in Ca compositions, with both Ca-saturated lherzolitic as well as low-Ca harzburgitic varieties represented. Compared to these chemistries the garnets analysed in the present study are quite calcic and compositionally almost at the maximum CaO-limit defined by garnet inclusions in C ° 5 diamonds from Premier. 7 1 C 1 ° 800 1200 0 0 2 1 ogen (ppm) C ° r 5 2 2 1 Total nit ° C 0 5 2 1 0 400 0 20406080100 Aggregation State (%N in B) Figure 4: Plot of nitrogen content and aggregation state Figure 3: Octahedral diamonds recovered from the for diamonds from the Premier xenolith. Isotherms Premier xenolith. calculated for a mantle residence time of 750 Ma. DIAMONDS REFERENCES All diamonds recovered are colourless. Most crystals are sharp-edged octahedra (Figure 3), some with minor Daniels LRM, Richardson SR, Menzies AH, De Bruin D, development of the dodecahedral form (this presumably Gurney JJ., 1995. Diamondiferous garnet formed on crystals partially protruding from the macrocrysts in the Newlands kimberlite, South xenolith which would have been in contact with the Africa – Rosetta stones from the Kaapvaal craton: Extended Abstracts 6th International molten kimberlite magma). A number of crystals are Kimberlite Conference. twinned octahedral macles, while aggregates of two or more octahedra are also common. Mineral inclusions Dawson JB, Smith JV., 1975. Occurrence of diamond in a are rare. Where present they are predominantly small mica-garnet lherzolite xenolith from black rosettes of sulphide. In one instance a kimberlite. Nature 254, 580 - 581. polymineralic (presumably lherzolitic) assemblage of Gurney JJ, Switzer GS., 1973. The discovery of garnets reddish garnet, green clinopyroxene and a colourless closely related to diamonds in the Finsch pipe. mineral is recognised. The xenolith is estimated to Contrib Mineral Petrol 39, 103 – 116. contain a minimum of 27 700 carat/ton diamond. 8th International Kimberlite Conference Long Abstract 2 Richardson SH, Harris JW, Gurney JJ., 1993. Three generations of diamonds from old continental mantle. Nature 366, 256 - 259. Shee SR, Gurney JJ, Robinson DN., 1982. Two diamond- bearing peridotitic xenoliths from the Finsch kimberlite, South Africa. Contrib Mineral Petrol 81, 79 - 87. Viljoen KS, Swash PM, Otter ML, Schulze DJ, Lawless PJ., 1992. Diamondiferous garnet harzburgites from the Finsch kimberlite, Northern Cape, South Africa. Contrib Mineral Petrol 110, 133 - 138. Viljoen KS, Robinson DN, Swash PM, Griffin WL, Otter ML, Ryan CG, Win TT., 1994. Diamond- and graphite-bearing peridotite xenoliths from the Roberts Victor kimberlite, South Africa. In: Proceedings of the fifth international kimberlite conference (1991), vol 1, Kimberlites, related rocks and mantle xenoliths. Eds HOA Meyer and OH Leonardos. Companhia de Pesquisa de Recursos Minerais – Special Publication 1/A Jan/94, Brasilia, 285 - 303. Contact: KS Viljoen, PO Box 82232, Southdale, 2135, South Africa, E-mail: [email protected] 8th International Kimberlite Conference Long Abstract 3.